I'm going to try to explain this but my english knowledge could be too limited.
A playback mode (desable by default) could make complex missions easier. Imagine making a F9 with recovery. With playback you could fly the F9 and control de 1st stage booster after separation, land it and start playback. When getting to the separation again change the control from the booster to the 2nd stage and see the booster maneuver itself.
Another function this could have (and I would love this) is content creation. Having a playback mode we could make more complex videos by first playing the mission and then caring about filming in playback, attempting different camera angles and improving the quality of them.
I think this could be implemented by saving the ship position, direction and burn time, without calculating again the fisics (that could end with different resoults each time and be more demanding for the PC).
I hope I've explained this clearly enough :)
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